Opal glasses have been produced for many years and have been used extensively in such applications as baking ware, lamp globes, and tableware. The mechanism giving rise to the opacity in such glasses is the presence of a second phase therewithin, this second phase, whether it be amorphous, crystalline, or simply voids, having an index of refraction different from that of the matrix glass.
Opal glasses have been generally categorized into two broad groupings; viz., spontaneous opals and thermally opacifiable opals. The former type develops opacity ("strikes in") as the molten glass is cooled to a solid body whereas the latter, frequently termed a reheat opal, contemplates cooling the molten glass to a solid body and thereafter heat treating that body to effect opacity therein. It will be apparent that, from the standpoint of cost of manufacture, spontaneously opacifiable glasses are preferred over glasses requiring a subsequent heat treatment.
Two classes of spontaneous opals are known to the art; viz., those wherein the opacifying phase is a crystal and those characterized by the separation of a glassy phase having a composition different from that of the parent glass and which is insoluble in the parent glass.
Borosilicate-type glasses typically exhibit relatively low coefficients of thermal expansion (25-50.times.10.sup.-7 over the range of 0.degree.-300.degree. C.) and, hence, demonstrate good resistance to thermal shock. This property has recommended their utility in applications where the glasses will be exposed to elevated temperatures, e.g., culinary ware.
U.S. Pat. No. 3,275,492 discloses alkali metal borosilicate opal glasses wherein the opacity may be developed spontaneously or upon reheating the glass body, the opacity resulting from the separation of an immiscible glassy phase. The glasses are stated to consist essentially, expressed in terms of mole percent on the oxide basis, of
______________________________________ SiO.sub.2 66-81 (equivalent to 62-72% by weight) B.sub.2 O.sub.3 10-27 (equivalent to 12-30% by weight) Li.sub.2 O and/or Na.sub.2 O and/or K.sub.2 O 1-7 ______________________________________
to which 3-34% of an opacifying agent is added selected from the group of BaO, CaO, CoO, CuO, MgO, NiO, and ZnO.
U.S. Pat. No. 3,661,601 also describes borosilicate-based, spontaneous opal glasses wherein the separation of an amorphous phase provides the desired opacity. The glasses consisted essentially, in weight percent, of:
______________________________________ SiO.sub.2 50-75 Al.sub.2 O.sub.3 3-9 CaO 11-20 B.sub.2 O.sub.3 1-7 F 2-4 Na.sub.2 O 0-7 K.sub.2 O 0-7 Na.sub.2 O + K.sub.2 O 3-10 BaO and/or SrO and/or P.sub.2 O.sub.5 and/or La.sub.2 O.sub.3 and/or TiO.sub.2 and/or Nb.sub.2 O.sub.5 and/or ZnO and/or GeO.sub.2 and/or PbO and/or Bi.sub.2 O.sub.3 and/or CeO.sub.2 0-10 ______________________________________
U.S. Pat. No. 3,723,144 discloses another borosilicate-based, spontaneous opal glass wherein opacity results from the separation of an immiscible glassy phase. Those glasses consisted essentially, in weight percent, of:
______________________________________ Li.sub.2 O 0.5-2.5 ZnO 7-10 B.sub.2 O.sub.3 11-14 SiO.sub.2 71-76 Na.sub.2 O 0-0.5 K.sub.2 O 0-3 Al.sub.2 O.sub.3 0-1.5 ______________________________________
U.S. Pat. No. 3,728,139 describes yet another borosilicate-based, spontaneous opal glass wherein opacity is provided by means of the separation of an immiscible amorphous phase. Those glasses consisted essentially, in weight percent, of:
______________________________________ SiO.sub.2 72-76 B.sub.2 O.sub.3 9-14 Al.sub.2 O.sub.3 0-1.5 Li.sub.2 O and/or Na.sub.2 O and/or K.sub.2 O 1-6 CaO and/or MgO and/or ZnO 3-9 ZrO.sub.2 0-0.5 TiO.sub.2 0.5-2.0 MoO.sub.3 and/or WO.sub.3 and/or As.sub.2 O.sub.3 1-3 ______________________________________
One application for which a borosilicate-based opal glass would be extremely useful is as a shelf in a microwave oven. Thus, the low coefficient of thermal expansion of the material would essentially eliminate concern regarding breakage resulting from thermal shock. To preclude undesirable heatup of the shelf during exposure to microwave, the alkali metal content of the composition will preferably be held at a low level.
There are numerous known methods for drawing glass sheet from a melt wherein the glass sheet does not come into contact with a mold or rollers until it has cooled sufficiently to resist surface marking. U.S. Pat. Nos. 3,338,696 and 3,682,609 disclose downdraw processes wherein careful control can be had in forming glass sheet of very uniform thickness and of optical quality.
Those processes, however, contemplate maintaining large volumes of glass at relatively low temperatures to obtain acceptable sheet-forming viscosities in the 10.sup.4 -10.sup.6 poise range. The molten glass will also be in long term contact with the refractory metals or ceramics which act as forming elements for the drawn sheet. Accordingly, those sheet-forming processes impose severe constraints upon operable glass compositions due to the glass stability and liquidus problems inherently associated with the handling and forming of molten glass at relatively low temperatures.
In summary, for utility in those sheet-forming processes, the glass must demonstrate a viscosity at the liquidus temperature of at least 10.sup.4 poises and, preferably, about 10.sup.5 poises and higher. Moreover, the glass melt must display extended stability against devitrification and interfacial crystallization in contact with refractory metals and ceramics such as platinum, sillimanite, mullite, zircon, and high density, alumina-containing refractories which have been used to contain and/or form molten glass. This stability must be maintained down to temperatures where the glass manifests a viscosity between about 10.sup.4 -10.sup.6 poises, the range of viscosities at which the glass is customarily drawn.
Finally, of course, to produce an opal glass wherein the opacity is of a uniform density, the opacifying phase must strike in very rapidly as the sheet is being drawn from the molten glass.
Therefore, the primary objective of the instant invention is to produce a spontaneous opal glass of dense opacity having a composition within the borosilicate system, which glass demonstrates a relatively low coefficient of thermal expansion, preferably less than 35.times.10.sup.-7 /.degree.C., and the capability of being used in elevated temperature applications, i.e., having a strain point in excess of 560.degree. C., the composition being low in alkali metal content such as to permit its use in microwave applications, and exhibiting the necessary stability and viscosity characteristics for drawing sheet in the downdraw processes described in U.S. Pat. Nos. 3,338,696 and 3,682,609, supra.